1. Field of the Invention
The present invention relates to surface acoustic wave devices used as, for example, band-pass filters or resonators and methods for manufacturing the surface acoustic wave devices. In particular, the present invention relates to a surface acoustic wave device including a plurality of wiring patterns having different potentials that are arranged on a piezoelectric substrate so as to three-dimensionally intersect with each other with insulating layers disposed therebetween and the present invention also relates to a method for manufacturing such surface acoustic wave devices.
2. Description of the Related Art
Surface acoustic wave devices have been widely used as band-pass filters or resonators. The surface acoustic wave devices are strongly required to have a reduced sized and therefore are likely to have a configuration in which a plurality of wiring patterns having different potentials are arranged on a piezoelectric substrate so as to three-dimensionally intersect with each other with an insulating layer disposed therebetween.
In some applications, the surface acoustic wave devices are required to have high power durability. Japanese Unexamined Patent Application Publication No. 2002-305425 discloses surface acoustic wave devices including electrodes having excellent power durability. With reference to FIG. 10, a surface acoustic wave device 1001 disclosed in Japanese Unexamined Patent Application Publication No. 2002-305425 includes a piezoelectric substrate 1002 made of a LiNbO3 or LiTaO3 single-crystal. In the manufacture thereof, after affected layers are removed from surfaces of the piezoelectric substrate 1002, a base electrode layer 1003 primarily including at least one of Ti and Cr is formed by vacuum vapor deposition. An Al electrode layer 1004 including Al or primarily including Al is then formed. The Al electrode layer 1004 is an epitaxial film having six-fold rotational symmetric spots appearing in an XRD pole figure and has a twin crystal structure. The Al electrode layer 1004 has a crystal orientation in which the crystallographic z-axis of the piezoelectric substrate 1002 substantially coincides with the direction normal to the (111) plane of an Al crystal. Japanese Unexamined Patent Application Publication No. 2002-305425 describes that grain boundaries in the Al electrode layer 1004 are not greater than one interatomic spacing and the self-diffusion of Al through the grain boundaries does not substantially occur. Japanese Unexamined Patent Application Publication No. 2002-305425 also describes that an Al—Cu alloy including a small amount of Cu may be used instead of Al.
The surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2002-305425 has increased power durability because of the use of the Al electrode layer 1004 which has the twin crystal structure. The inventors of the present invention have verified that in the case of adding Cu to Al, Al electrode layers having a higher Cu content have higher power durability. Surface acoustic wave devices having a Cu content of 0%, 0.1%, or 1% have been manufactured and have then been tested for power durability in an acceleration mode (power greater than power usually used is applied). FIG. 11 shows the test results. MTTF shown in FIG. 11 refers to a mean time to failure (h). Power is a value that is normalized based on the power that causes a failure of the surface acoustic wave device having a Cu content of 0%. A reduction in loss of 0.3 dB was classified as a failure.
As shown in FIG. 11, an increase in Cu content increases the mean time to failure, which increases the power durability.
For the purpose of size reduction, surface acoustic wave devices are likely to have a configuration in which interconnections connected to different potentials three-dimensionally intersect with each other with an insulating layer disposed therebetween as described above. When a surface acoustic wave device having such a configuration includes an electrode layer that is made of an epitaxial film of an Al—Cu alloy prepared by heavily doping Al with Cu for the purpose of improving power durability and which has a twin crystal structure having six-fold rotational symmetric spots appearing in an XRD pole figure, there is a problem in that hillocks or protrusions are formed on IDT electrodes.
When using a polycrystalline Al—Cu alloy instead of an Al—Cu alloy film made from an epitaxial film having a twin crystal structure having six-fold rotational symmetric spots appearing in an XRD pole figure, hillocks or protrusions are not formed. When using the epitaxial film, which is made of the Al—Cu alloy prepared by heavily doping Al with Cu for the purpose of improving power durability and which has a twin crystal structure, abnormal hillocks or protrusions are likely to be formed on the IDT electrodes during heat curing of the insulating film to form the three dimensionally intersecting portion.